Inside The Earth [NU009]
Ben Franklin Centre for Theoretical Research
PO Box 27, Subiaco, WA 6008, Australia.
"The goal of scientific endeavour is to learn the truth of nature,
and not to win debates"
Man lives, together with the complex assembly of plant and animal life which make up the
biosphere, on the surface of the Earth. So it is natural that we know most about the surface,
which is a surprisingly thin piece of real estate. When we go beneath the surface, down into
the depths of the Earth, we can make direct observations for only about one-fifth of one percent
of the way down. Everything we know about the other 99.8% has had to be deduced from
indirect evidence and assumption and calculation -- so it is again natural that the picture we
have may be inaccurate.
In fact almost everything we have worked out about the insides of the Earth comes from
a study of one thing -- earthquake waves. When an earthquake occurs, it produces shock
waves which run through and around the Earth.
These waves do not just radiate out and dissipate, they actually bounce off various levels
within the Earth, called discontinuities, where the physical properties of the rocks change.
Observations of the waves reveal just where under the surface the discontinuities are located,
and give an indication of the properties of the rocks on either side.
Fig. 9.1. Structure of the Earth, from surface to centre
Figure 9.1 shows a conventional summary of what has been deduced about the properties
of the Earth, from its surface to its centre (about 6370 km down). A fuller explanation can be
found in any modern text (eg ), but the main assumed features are these:
Feature A Four discontinuities, at depths of around 20, 200, 2900, and 5150 km, where
there are abrupt changes in density;
Feature B Density increasing from about 2.7 (g/cc) at the surface to about 13.6 at the
Feature C Assumed chemical composition changing from oxides high in silicon and
aluminium (Sial) or high in silicon, magnesium, iron and calcium (Sima) at the surface, down
through layers with increasingly less silicon and more iron, until a core region is reached
containing mostly iron oxides;
Feature D Temperature increasing from around 20 (ºC) at the surface to 3000 at the
Virtually everything in the area of domainography which has been mentioned so far
concerns the region called the lithosphere, which lies beween the uppermost solid surface and
a depth of around 60-100 km.
This region is made up of the Crust and the uppermost division of the Mantle. The
boundary between these two is the first discontinuity, called the Moho (after Mohorovicic,
who discovered it). There the density changes abruptly, from 2.9 to 3.3.
The Moho varies in position from about 10 to 35 km below sealevel. Under the seabeds,
where the oceanic basic rock or Sima is exposed, it is at its shallowest, while it is furthest down
under the continents, which consist mostly of continental acidic rocks, or Sial. Where Sial
exists, in the continental areas, it is nevertheless believed to have Sima underneath it, so the
Sima extends over the whole of the Earth. The Moho also extends over the whole Earth,
apparently almost entirely within the Sima.
There is no doubt that the Moho, and the other three discontinuities mentioned in Feature
A, all exist. What is not certain is what their nature is. They have been associated with changes
in chemical composition to some extent, but this is purely speculative. Even the Moho is only
just within the range of feasibilility for reaching by modern drilling techniques (the current
record depth is around 11 km), but to date no drill core recovered has provided hard evidence
that rock below the Moho is of different composition to that above.
From all the evidence produced to date, it is clear that extensive domain movements have
occurred in the past, and it seems that the influence of these movements has extended at the
very least to the base of the lithosphere, up to 100 km down. On the old plate tectonic theory,
the 'plates' were supposed to be drifting on the mushy rock layer, the asthenosphere,
immediately below the lithosphere.
It is interesting that as long ago as 1782, Benjamin Franklin speculated that "the surface
of the globe might be a shell, capable of swimming on an internal fluid". We will see later that
this attractive idea, which has now actually reached general acceptance, is false.
It seems logical that if such 'floating' movements occurred, they could not leave intact a
boundary based on differences in chemical composition. Even on Continental Drift principles,
it can be taken for granted that one such 'chemical boundary', that between the Sial and the Sima, would have already been broken up by domain movements.
It therefore seems likely that the Moho is a physical boundary. Its most probable nature
is that of a phase change, brought about by increasing pressure. A given mineral or compound
can exist in different states or 'phases', caused by the application of external pressures -- an
example is diamond and graphite, both of which are phases of carbon. Diamonds can be made
from graphite by application of very high pressures -- amusingly enough, they have actually
been made from peanuts, after extracting the carbon from these.
The Moho discontinuity represents a phase change boundary
where the rocks are changing their phase in response to increasing
From this Proposition it follows directly that if the Moho is dependent on pressure, it will
alter its position as the pressures change, as will happen when domains move and break up.
The position of the Moho will change as the pressure of
overlying rock changes in consequence of domain movement
These propositions are in accord with Features A and B above, but Feature C looks a bit
more shaky. Let us now look further at the matter of chemical compositions.
Internal Chemistry of the Earth
Once we descend below the 10 km or so of the Crust which can be analysed directly, our
knowledge of the Earth's chemistry is purely speculative. So we are quite within our rights
to question some of the assumptions made, assumptions often repeated as fact, from textbook
to textbook through the decades, without any real evidence.
One of these 'facts' is that the core of the Earth is very rich in iron and poor in silicon.
According to the text used to construct Fig. 9.1 , some 90% of the core consists
of iron oxides, and another 8% of nickel oxide, leaving 2% unspecified. Where did this idea
Well, firstly there is the fact that the core material is very dense, and both iron and its oxides
are also dense (though not as dense as the core material). So this does not get us far. Perhaps
the strongest argument comes from material assumed to be derived from outside the Earth,
Meteorites are solid objects falling to the Earth from space, of a size great enough to survive
vaporization through the friction of falling through the atmosphere (another instance of the great heat available from friction). They are of two types, either 'stony' (made of rocks similar to those found on Earth), or 'iron' (not similar to any Earth-surface rocks).
It has been suggested that meteorites are the remains of a planet similar to Earth which
broke up at some time in the past. The proportion of stony to iron meteorites falling on Earth
is similar to the proportion of iron to stone within the Earth if its core was made of iron. And
this appears to be the main argument for an iron core.
This connection is not only tenuous, it is also flawed. The iron in meteorites is native metal,
while the iron assumed to be in the core is represented as iron oxides. No mechanism has ever
been suggested whereby, if a planet broke up, the bits of iron oxide from its core could all be
converted to iron metal.
There is one further possible link, with magnetism. The Earth has an considerable magnetic
field, and iron and its compounds are normally linked with magnetism. However, any possible
connection with the core is ruled out by the fact that magnetic materials lose their magnetism
when heated up -- this is the basis of the technique of paleomagnetism, mentioned in NU003, whereby newly-created rock from volcanos took on the direction of the local magnetic field
as it cooled down.
So it appears that the idea that the Earth has an iron-rich core is without firm basis.
The Earth does not have an iron-rich core
What then is the basis for the density discontinuities in the Earth's inner structure? I think
it is reasonable to assume that all four of the known discontinuities mentioned in Feature A
are, like the Moho, the result of pressure-induced phase changes.
The four discontinuities marking the boundaries between the
Earth's Crust, Upper Mantle, Lower Mantle, Outer Core, and
Inner Core are all due to pressure-induced phase changes
This Proposition is at least as reasonable as any other. It is supported by the manner in
which masses of molten rock material segregate. Some segregation might be expected,
perhaps with lighter components rising closer to the surface (as with the Sial and Sima), but
not a sharp division based on chemical composition.
A corollary of Proposition 9D is that all pressure-dependent discontinuities may be
expected to alter their position as the Earth expands, as suggested for the Moho.
All the density discontinuities within the Earth may be expected to change position as internal pressures change with Earth expansion
Heat of the Earth
Of the four Features listed above, the one most open to attack is the one which assumes that
the Earth's temperature increases continuously towards its centre. After all, why should it?
When confronted with this question, probably the responses of most geophysicists would
fall into two areas. One is to say, whatever the reason for the phenomenon, it describes known
behaviour -- it does get hotter as you go downward in the Earth. The other response might
be to say that pressures are very great within the Earth (this is undisputed), and high pressures
and high temperatures are usually associated.
Neither of these responses hold much water. The most obvious manifestation of heat from within
the Earth is through geothermal phenomena such as volcanos. We have already seen that these
essential local phenomena are, in fact, an expected outcome of domain rubbing (Proposition 8D).
Well then, what about the observed fact that temperatures increase as you go downwards
in mines? This is perhaps the strongest argument which can be produced, and we need to look at it more closely.
Temperatures down mines
As you go progressively deeper in the Earth, temperature variations due to seasonal and
climatic effects smooth out, and in an undisturbed site the temperature is virtually constant
when a depth of 20-30 m is reached. Of course this constant temperature is different for
different sites and depths.
As you go deeper still, the temperature invariably rises regularly. The rate of rise, at least
for the more shallow mines, is around 1ºC for each 40 m of descent (equivalent to 25ºC/ km),
but it does vary by a factor of two, and even varies at different depths in the same mine. This
effect has been known since mines were first dug.
More recently, information has been gained from deep oil wells, showing a similar picture
. Producing oil wells have been drilled, in different parts of the world, down
to depths of around 8 km. The temperature of the oil produced varies according to the area of
production and the geology of the host rocks, but invariably increases with the depth of origin.
Oil from a depth of 1 km might be at a temperature of 55ºC, that from the deepest wells could
reach 240ºC. Again these figures represent rises of around 25ºC/ km.
The pertinent question now is whether these rises increase regularly as you go even deeper,
and if so, what the source of the deep heat is. For the first part, a regular rise of the type found
in the accessible top 10 km implies a temperature at the Earth's core of over 150,000ºC. This
is the sort of temperature believed to exist only in the interiors of very hot stars, and no one
has ever suggested it as a real possibility for the Earth -- 3000ºC is a conventional figure.
This last figure represents an average rise of less than half a degree per kilometre, so even
on the conventional view it is accepted that any rate of rise must fall off as you go deeper.
For the second part, the accepted origin of the heat rising from the Earth's interior is that
it comes from the molten core, presumed to be left over from when the planet was first formed
in a molten state. We now look at the flow of this heat.
Heat Flow in the Earth
The matter of the flow of heat through the body of thy Earth has received considerable
attention in the past. Rocks are poor conductors of heat, and so if the centre of the Earth was
once very hot, it can be expected that it would take some time for the inner heat to escape. But
just how long?
Towards the end of the last century, the famous physicist Lord Kelvin (William Thomson)
looked at this problem in an attempt to work out the age of the Earth. Kelvin was an expert
in matters relating to heat flow, in fact the absolute scale of temperature, that starting from
absolute zero (around -273ºC), is measured in degrees K (K for Kelvin).
Kelvin concluded that if the Earth started off very hot, it could not take more than 400 my
for it to cool down to its present temperature, and it could be as little as 20 my. His study was
one of the earlier scientific attempts to work out the age of the Earth, and his result of 20-400 my
was accepted as valid. It had already been realised that previously assumed values, of a few
thousands of years only, were far too low, but the full extent of the Earth's age had not yet been
After the discovery of radioactivity, it was realized that the radioactive decay of elements
in rocks provided a way of working out their age. Many elements have forms, called
radioactive isotopes, which are unstable, that is they break down to form other elements and
give out radioactivity. For example, a uranium isotope may change slowly into lead.
Each radioactive isotope decays at a fixed rate, called its half-life (the time taken for half
of all the original isotope atoms to change). This half-life may vary, for different isotopes,
from many thousands or millions of years down to a fraction of a second. Isotopes with long half-lives
can be used to work out ages of rocks. This is done by analyzing the amount of a particular
radioisotope in a mineral and the amount of the element it is changing into -- the ratio and the
half-life give the time since the original mineral was formed.
What this method showed, when applied to rocks, was that most of them were very much
older than Kelvin's result suggested. Confirmation of this great age came from other results
also, so the Earth was clearly too old for its internal heat to be 'left over' from when it was first
The next suggestion for the origin of this heat again was connected to radioactivity. When
radioisotopes decay, they give off heat -- it is this heat which is used in nuclear power stations.
All rocks contain a larger or smaller amount of radioactive material, and a currently suggested
cause for the Earth's heat is that it stems from the decay of radioactive material within the crust,
perhaps at depth.
There are problems with this theory. One is that the rocks which are richer in
radioactive material are known to be concentrated in continental-type rocks, rather than
oceanic ones. Upward heat flow does vary in different parts of the Earth, but not in a manner
which has any relationship with content of radioactive materials. Another problem is that the
content of radioactive material in the rocks is quite insufficient to produce the amount of heat
actually recorded, especially since the evidence is that concentrations of radioactives, always
relatively uncommon, are mostly confined to the topmost continental Sial layer of the Earth.
The Encyclopaedia Britannica article on the Earth's heat  concludes that "no satisfactory
theory ... yet formulated explains the Earth's thermal constitution". In
summary, it appears that there is no convincing evidence that the core of the Earth is very hot.
The core of the Earth is not especially hot
Where then does the observable heat flow originate? We already have an answer -- from
domain movements. In this case, however, we probably need to include the deeper domains,
down to a depth of around 100 km. We will see later, in NU016, that there are other possible
contributions to this heat, but this is suggested as the main source.
The principal source of the heat observed to flow from the
depths of the Earth is friction from movement of domains,
including deeper domains
Is the Earth Made of Brown Sugar?
The Earth is not made of any sort of sugar. But in some respects we get a better intuitive
picture of what is happening with the Earth if we regard it as made of a substance similar to
sugar crystals, rather than of a baked-clay material as implied by the term 'plate'.
The hardness and physical strength of various rock materials are readily observed and
measured, and so people have an intuitive feel for how a boulder or pebble will behave. There
is a difficulty when this feeling is extended to something on quite a different scale, such as a
mountain range. The behaviour of a microdomain is not the same as that of a boulder of the
same material, scaled up in proportion.
An instance of this has already been looked at, in Rule 3 of NU007, where it was noted
that rotation of domains was not normally observed, say when one collided obliquely with
another. What actually happens in such an impact is that the material at the point of impact
just crumples up, like a mass of sugar crystals, rather than a whole 'plate' swinging about the
point of first contact.
The sugar picture also gives a better feel for what happens with deeper domains and under-domains.
If a surface domain is split apart by Earth expansion, its edges will be 'soft and
crumbly' on the larger scale, and if sub-surface domains split, like sugar cubes embedded in
a mass of crystals, the higher material will tumble down into the gaps.
Fig. 9.2. A domainographic representation of the Earth's upper layers
Figure 9.2 is a representation of how surface and subsurface domains might be represented
in a cross-section of the top 100 km of the Earth's surface. If they are looked at as lumps and
cubes of sugar in a sugar crystal matrix, this may give a better feel for the situation than one
showing them as single rigid flat plates floating on a molten surface (the conventional picture).
Another image might be that of a dry-stone wall, pieces of rock fitted together with only
smaller rock fragments and no mortar in the gaps.
Another advantage of these pictures is that they make it easier to visualize the independent
movement of domains and sub-domains at different levels. The old tectonic plate image only
considers movement of one-layer massive plates of more or less uniform thickness, around
Having looked inside the Earth, we now return to its surface, but this time we look not at
the solid land, but at its rolling oceans.
* * * * * * * * * * * * * * * * * * * * * * * * * * * * * *
(Full list of references at NURefs)
. Peter Bergman. Personal communication. 1989.
. Encyclopedia Britannica. 1974 edition. Vol.6 p26.
. The Physical Earth. Mitchell Beazley.
NU010: The Rolling Oceans
NU008: Making Mountains Out Of Movements
Go to the NUSite Home Page
Version 1.0, printed edition ("Nuteeriat: Nut Trees, the Expanding Earth, Rottnest Island, and All That...", Planetary Development Group, Tree Crops Centre, 1989).
Version 2.0, 2004, PDFs etc on World Wide Web (http://www.aoi.com.au/matrix/Nuteeriat.htm)
Version 3.0, 2014 Sep 23, Reworked from Chapter 9 of "Nuteeriat" as one article in a suite on the World Wide Web.